Dental caries, bone diseases and bone fractures are some of the most common ailments in all societies. For example, almost 40% of UK adults have at least one untreated carious tooth, and this rises to 60-90% for children or across the whole population in deprived nations. Further, currently 50-60% of postmenopausal women will suffer an osteoporotic fracture, commonly of the hip, vertebra or wrist. Conventional composites and cements that are currently used to treat these diseases have a number of disadvantageous properties, which are discussed below. A great need therefore exists for improved materials to treat those in need of treatment.
Materials which are initially liquid and mouldable but that can set rapidly in situ giving immediate structural support and adhesion to surrounding tissues are of great value in bone tissue-engineering applications as well as dental, maxillofacial and orthopaedic surgeries. As they set from liquid to solid, micromechanical bonds are formed with the surrounding surfaces. The adhesive effect is especially strong with rough surrounding surfaces.
Setting of such materials may, for example, be initiated by chemical initiators or by exposure to visible or UV light (especially in cases of chemical polymerisation and cross-linking, such as in double bond containing (eg. methacrylate) polymeric formulations), or may be a result of other chemical reactions upon mixing of two components (eg acid/base in glass ionomer dental and brushite-forming bone cements) or solvent removal or evaporation from the initial liquid formulation.
For example, injectable methacrylate based dental restorative composites, adhesives and poly(methyl methacrylate) (PMMA) bone cements have been widely used for applications such as tooth restoration and for fixing of orthopaedic implants. After injection of the initially fluid formulation (containing various methacrylate monomers and inorganic particles or PMMA powder in combination with liquid methyl methacrylate monomer), curing occurs, due to the presence of chemical initiators, and results in a solid material.
As mentioned above, conventional (e.g. methacrylate-based) composites and cements have a number of disadvantageous properties. The PMMA cements and dental restorative composites discussed above are strong, but curing of large volumes generates excessive heat and material shrinkage which may cause necrosis of surrounding tissue or debonding. Additionally, if setting is slow, release of potentially toxic un-polymerised compound is a problem[1,2]. PMMA also causes potential long-term biocompatibility problems due to production of wear particles [3].
Dental caries is associated with bacteria such as Streptococcus mutans and S. sorbrinus that, through acid production, cause hydroxyapatite dissolution and enamel/dentine demineralisation. Subsequent proteinase action degrades remaining dentinal collagen. Damaged or infected tooth structure must be partially or totally replaced in order to halt disease progression, preserve function and improve aesthetics. Due to mercury concerns and patient demand, amalgam restorations are increasingly being replaced by more aesthetic materials such as composites. Composites have high strength, but their main reason for replacement is bond damage and recurrent caries
Antibacterial agents may be incorporated into dental composites or antibiotics into PMMA bone cements to decrease the risk of infection [13, 14, 16]. However, the release from conventional composites and cements is restricted, and requires high drug content which usually decreases material strength.
Metal screws, pins and plates are increasingly used in arthritic joint replacement or osteoporotic fracture repair due to rising population age and disease incidence. Titanium alloys are often employed due to their biocompatibility. High strength is required to enable use of thin plates and finer screws/pins. For a given material type, maximum strength is usually proportional to modulus but high modulus causes bone atrophy. Titanium screw strength is typically only 1-2% of the modulus [15]. Other problems can include infection (osteomyelitis) and screw loosening particularly from bone weakened by osteoporosis. Titanium screws and pins can be designed to encourage hydroxyapatite (HA) deposition which encourages osseointegration. Alternatively, early metal fixation may be improved by combining screws/pins with bone cements (screw augmentation/bone filling).